Field of the Invention
The present invention relates to a process for producing a toner for developing electrostatic latent images. Toner for electrically or magnetically developing latent images is used in a variety of image forming processes.
A method of electronic photography is disclosed in U.S. Pat. No. 2,297,692 in which photoconductive material is usually employed to form electric latent images on a photosensitive paper in a variety of ways. The thus-formed latent images are developed using a toner to form a toner image which is transferred to a material such as paper when required. Next, this toner image is fixed to the above material by employing heat, pressure, both heat and pressure, or solvent steam so that a copy can be obtained. A variety of types of developing methods have previously been disclosed in which toner is used, as well as different methods of fixing toner images, and a method is selected which is most suitable for the particular image forming process being employed.
Hitherto, the toner used has generally been prepared in such a manner that a coloring material comprising a magnetic body, a color or a pigment is dissolved and mixed into a thermoplastic resin so as to be uniformly diffused therein. The mixture is then pulverized and classified so that a toner having a desired particle size is prepared.
With this preparation method (pulverization), a relatively desirable toner can be prepared, although certain limitations are involved regarding the selection of the material to be used for the toner. For example, once the melted and kneaded substance (the dispersed body of resin and coloring material) has cooled, it must be sufficiently brittle so that it can be pulverized by an economy pulverizer. However the dispersed body must be sufficiently brittle so that, it is easy to form a group of particles having an excessively large particle diameter distribution range at the time of pulverization. A particular problem may arise if particles which have been excessively pulverized are present in this group or if the brittle material could be easily pulverized or powdered even smaller when it is ultimately used in a copying machine or a page printer for the purpose of developing an image.
In the process of pulverization, it is relatively difficult for solid grains such as magnetic powder or coloring material to be uniformly dispersed within the resin. Since the degree to which these solid particles are dispersed may lead to an increase in the degree of fogging or a reduction in the image density, great attention must be paid to the degree of dispersion of the magnetic powder or coloring material. Furthermore, if the coloring material is located on the surface of the powder, it will be crushed and cause a change in the toner developing characteristics.
On the other hand, in order to overcome the problems experienced with such a pulverizing method, some suspension polymerization methods of producing toner were disclosed in Japanese Patent Publications Nos. 36-10231; 43-10799 and 51-14895. In these suspension polymerization methods, monomer compositions obtained by uniformly dissolving or dispersing a polymerizable monomer and a coloring material (where an initiator of polymerization, a crosslinking agent, a charge controller, and other additives may be added if necessary) are immersed in an aqueous phase. The monomer composition is then granulated while being stirred so that it can be polymerized in order to form toner particles.
Such suspension polymerizing methods are preferred since the toner material does not need to be brittle and exposure of the coloring material on the surface of the toner is prevented since there is no pulverizing process. It also exhibits the advantages that adequate fluidity can be obtained since the shape of the obtained toner particles is spherical and the friction electrification is uniform. However, it is a complex undertaking to perform polymerization in a stable suspension system while preventing particles of the monomer composition from aggregating while obtaining uniformly small polymerized particles.
The various characteristics of the toner used in a method of developing latent images need to be controlled to allow the original image to be faithfully reproduced. In this regard, it is critical to control the particle distribution. If an image is formed using a toner having a wide particle distribution, thereby including a large quantity of particles having disordered particle sizes, the toner may be scattered or foggy, or non-uniform images may be generated, leading to deteriorated image sharpness. Furthermore, since the developing characteristics of the toner cannot be made uniform, a problem arises in that the durability of the toner is poor.
The reason that particle size distribution is scattered is that each of the additive materials are uniformly diffused in the monomer compound. In particular, the diffusion of the coloring material, which is usually a necessary component, within the monomer composition greatly affects the particle size distribution. Therefore, a variety of ways to improve its diffusion characteristic have been disclosed. For example, it was disclosed in Japanese Patent Laid-Open No. 54-84731 that the diffusion characteristics of magnetic powder within a polymerizable monomer composition can be improved by bringing the magnetic powder into a lipphilic material. It is also disclosed in Japanese Patent Laid Open No. 56-116044 that the diffusion characteristics of carbon black in a polymerizable monomer composition can be improved by subjecting it to a graft treatment. Although the diffusion characteristics of each of the additives in a monomer composition can be improved by subjecting them to a hydrophilic treatment, a toner having a particle size distribution which is sharp enough for practical use cannot be easily obtained in this way.
On the other hand, a method of making toner particle size distribution sharp by improving the diffusion medium has been disclosed. For example, the method disclosed in Japanese Patent Laid-Open No. 57-42052 controls the particle size by combining a dispersant and an anion type of surface-active agent and Japanese Patent Laid-Open No. 56-156839 controls particle size by adding an aqueous polymerization inhibitor. However, there are certain aspects which remain unsatisfactory in terms of obtaining a polymerized toner having a preferred particle size distribution which exhibits good development characteristics. In particular, since surface-active agents such as an anionic surface-active agent cannot easily be removed by washing with water, such agents tend to remain on the surface of the toner particles, leading to deterioration of the toner developing characteristics.
A method of producing toner by suspension polymerization is disclosed in Japanese Patent Laid-Open No. 59-1238562 (which corresponds to U.S. Pat. No. 4,592,990) in which a monomer system including a cationic polymer and an anion type of dispersant are combined. In this method, the cationic polymer is made present on the surface of the droplets of the monomer composition while the anionic dispersant is present in the interfacial boundary of the anionic dispersant in the manner of an ionic combination by having the cationic polymer contained in the monomer composition, and polymerization is performed in an aqueous medium in which the anionic dispersant is present. As a result, a stable suspension granulation system can be formed to allow the production of a toner having a particle size of approximately 10 to 20 .mu.m with a sharp particle size distribution.
In order to obtain an even more stable suspension granulation system, in Japanese Patent Laid-Open No. 60-57856 an anionic, inorganic dispersant and a cationic, inorganic dispersant are mixed together. This allows the production of toner in which the generation of small suspensions is better prevented with respect to the sole use of an anionic, inorganic dispersant system. In this toner producing method, since the anionic inorganic dispersant and the cationic inorganic dispersant form substantially stable aggregates in the dispersed medium, and it is assumed that the stable aggregates cover the surface of the suspended particles of the polymerizing composition including the cationic substance so as to prevent generation of the particle toner.
There is a preferred method of producing small particle sized toner in a suspension polymerization in which high speed rotation is performed at the time of granulation by a device such as a homogenizer exhibiting high shearing force. In the above-described method of producing toner disclosed by the applicant of the present invention in Japanese Patent Laid-Open No. 60-57856, the stability of the suspended particles with respect to high rotation is insufficient and excessively granulated particles can be generated, resulting in a particle size distribution that is slightly excessive.
As described above, there are a variety of requirements for toner, in particular the ability to provide high grade images, a good quality developing performance, having a small particular size of less than 10 .mu.m and a sharp particle size distribution.